The present invention relates to power transmission units for motor vehicles, and more particularly to an improvement of a gear-shift mechanism for power transmission units.
Power transmission units are known of the type which includes input and output shafts supported in parallel to each other from a housing assembly for the unit, an idler shaft supported in parallel with the shafts from the housing assembly, driving and driven gears respectively mounted on the input and output shafts, an idler gear axially slidable on the idler shaft, a fork shaft slidably supported in parallel with the idler shaft from the housing assembly and being operatively connected to a manual shift lever to be selectively shifted from its neutral position to a forward or reverse position, a detent mechanism mounted on the housing assembly for selectively retaining the fork shaft in its neutral and shifted positions under a resilient load acting thereon, a shift head member slidable on the fork shaft, a one-way engagement mechanism for effecting engagement of the shift head member with the fork shaft in axial movement of the fork shaft to and away from its reverse position, and a shift arm member pivoted to the inner wall structure of the housing assembly and linked with the shift head member, and in which the idler gear is carried by the shift arm member to be retained in its neutral position and to be engaged with the driving and driven gears in the axial movement of the fork shaft to its reverse position.
In such power transmission units as described above, the detent mechanism acts to define the neutral and shifted positions of the fork shaft, and axial clearances are provided within the one-way engagement mechanism to avoid assembling failure caused by error in the manufacturing processes. For this reason, if the axial clearances result in interference between the idler gear and its adjacent gears or in vibrative displacement of the shift head and arm members and the idler gear in their neutral positions, there will occur unpleasant noises in operation of the transmission unit. In case the axial clearances result in insufficient return of the shift head and arm members to their neutral positions in shifting operation of the fork shaft, complete disengagement of the idler gear from the driving and driven gears may not be effected. It is also presumable that if the axial clearances result in insufficient shift of the shift head and arm members to their reverse positions in shifting operation of the fork shaft to its reverse position, complete engagement of the idler gear with the driving and driven gears may not be effected, causing undesired wear of the gears.